Data too large for tiny index in Kibana

I have a tiny index (400kB) that I try to do "Discover" on in Kibana. It gives me the error:
Data too large, data for [TimestampUtc] would be larger than limit of [2508954009/2.3gb]]

How can this be? I'm currently increasing the cluster size to overcome this, but there seems to be no reason to use over 2,3gb to store 800 documents...

Cluster 376b3a

It probably means fielddata, is that field analysed in your mappings?

Thank you for your answer.

No, it is just a date field, not analyzed:

"TimestampUtc": {
"type": "date",
"format": "epoch_millis||dateOptionalTime"

But even if it was, 400kb of data could not fill 2,3gb?